Description

The five attached screenshots do a good job of explaining part of the problem. Here are reproduction steps for a more complete representation:

1. /clear
2. Put 2 stacks of stone in your inventory and 2 stacks of stone in your hotbar
3. Grab any other item, perhaps a grass block, and shift-double-click one of the stacks of stone in your inventory with it

Expected: 4 stacks of stone in the hotbar. Actual: 4 stacks of stone in the inventory.